I. Field
The present invention relates generally to communication, and more specifically to channel estimation for a communication system.
II. Background
In a wireless communication system, a radio frequency (RF) modulated signal may travel via a number of signal paths from a transmitter to a receiver. If the signal paths have different delays, then a received signal at the receiver would include multiple instances of the transmitted signal at different gains and delays. This time dispersion in the wireless channel causes frequency selective fading, which is characterized by a frequency response that varies in magnitude and phase across the system bandwidth.
An accurate estimate of a wireless channel between the transmitter and the receiver is normally needed in order to effectively receive a data transmission sent by the transmitter. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of modulation symbols that are known a priori by the receiver, a channel response may be estimated as a ratio of the received pilot symbols to the transmitted pilot symbols.
Pilot transmission represents overhead in the system. Thus, it is desirable to minimize pilot transmission to the extent possible. This may be achieved by sending a “narrowband” pilot on a small portion of the system bandwidth and using this pilot to obtain a channel estimate (e.g., a frequency response estimate or an impulse response estimate) for the wireless channel. Regardless of whether the channel estimate is in the time domain or the frequency domain, the resolution of the channel estimate is generally limited by the bandwidth of the pilot used for channel estimation. Thus, a channel estimate with only coarse resolution may be obtained from a single transmission/instance of a narrowband pilot. This coarse channel estimate may provide poor performance if used by the receiver to recover the data transmission.
There is therefore a need in the art for techniques to derive a channel estimate with good resolution based on a narrowband pilot.